Looking back and with hindsight, a lot was going on and (as we now know) major changes, such as SWT losing the franchise, were not foreseen of course in the early days of the original 10 car suburban plans. These very complex plans, once commenced, will often continue to their natural (originally planned) conclusion, even if during the transition period other (as said) major changes come about for whatever reason. One contractural document relating to the first part (458 conversion to 5 car and proposed usage there of etc etc) is dated December 2011, and I suspect the process for acquiring further 5/10 car new build stock (707's) followed on very shortly after that. I can confirm that preliminary unit diagrams for that 'new build' stock, which eventually became class 707, had been planned and distributed to those involved in the project during 2013.